In high voltage electrical cables and machines, insulation breakdown is a concern at locations where the electrical potential across the insulation is high, such as at connection interfaces and areas where the insulation is surrounded by air. For example, corona discharge is an electrical discharge caused by the ionization of a gas, such as air, surrounding a conductor. Corona discharge occurs when the local electric field exceeds a threshold value. The threshold value is dependent on various conditions, such as distances between conductive elements separated by the gas and the electric field strength (e.g., the magnitude of the potential gradient), and the electrical breakdown strength of the fluid. Corona discharge can degrade the insulation layer, reducing the usable lifetime of the insulation layer, and can also generate ozone that is detrimental to the environment.
Stress control structures can be used to control the electrical field within electrically insulating structures as well as on free surfaces, reducing the risk of insulation degradation and breakdown. For example, conventional stress control for rotating machines is accomplished by wrapping insulation in one or more tapes of various conducing and non-linear electrical properties. However, taping has several inherent disadvantages. One issue with tapes is the risk of creating air pockets between adjacent layers of tape, as the air can result in corona discharge. Another issue with tapes is the difficulty in accurately controlling the geometrical structure and the electrical properties of the resulting stress control structure because of generally imprecise wrapping processes used to apply the tape and/or generally imprecise manufacturing processes to produce the tape. Therefore, it may be difficult to optimally control the electrical field concentration using conventional stress control techniques.